(1) Field of the Invention
The present invention relates to an optical pickup device in which a tracking error and a focusing error are detected based on a reflection beam of a light spot on an optical disk in order to carry out recording, reproducing or erasing of information of the optical disk.
(2) Description of the Related Art
Generally, in an optical pickup device, a tracking error detection and a focusing error detection are carried out. The tracking error detection allows the emission beam to trail a track of the optical disk to the exact position when recording, reproducing or erasing information of the optical disk. The focusing error detection allows the light spot to be accurately focused on the optical disk when recording, reproducing or erasing information of the optical disk.
There is a demand for mass-storage optical disks which are rewritable. In order to keep up with this demand, two main types of optical disk drive currently exist: a write-once optical disk drive which accesses a CD-R (compact disk recordable), and a rewritable optical disk drive which accesses a DVD (digital video disk). The optical pickup device of the write-once optical disk drive reads data from the CD-R, and can write data to the CD-R once only. The optical pickup device of the rewritable optical disk drive reads data from the DVD, and can write or erase data to the DVD many times.
The CD-R (which will be called a second optical disk) has a transparent substrate which is about 1.2 mm thick, and the DVD (which will be called a first optical disk) has a transparent substrate which is about 0.6 mm thick. In a standard CD-R type pickup device, a laser diode which emits a laser beam having a wavelength of 785 nm is used as a light source (which will be called a second light source). In a standard DVD type pickup device, a laser diode which emits a laser beam having a wavelength of 650 nm is used as a light source (which will be called a first light source).
As disclosed in Japanese Laid-Open Patent Application No.6-259804, an optical pickup device which is designed to access information of the first optical disk and the second optical disk in a shared manner is known. In the optical pickup device of the above publication, the first light source and the second light source are used.
On the other hand, it is known that a laser beam emitted by a laser diode is divergent, and the diverging angle of the laser beam is the maximum in a direction perpendicular to the recording layer of the optical disk and it is the minimum in a direction parallel to the recording layer of the optical disk. Generally, a cross-sectional profile of the laser beam emitted by the laser diode has a far field pattern (FFP) which is elliptic.
In order to obtain good characteristics for reproducing, recording or erasing of information of an optical disk, it is desired that the light spot on the optical disk, formed by the emission laser beam, is circular in cross section. It is likely that the reproducing, recording or erasing characteristics of an optical pickup device will be lowered as the light spot on the optical disk is deviating from the circular cross section into an elliptic form. Hence, a beam profile correcting device which suitably eliminates such a deviation from the light spot on the optical disk, formed by the emission laser beam, is necessary in order to obtain a light spot in the circular cross section for each of the first and second optical disks.
However, the optical pickup device of the above publication does not take account of the possible deviation of a light spot on the optical disk in the circular cross section. Hence, the reproducing, recording or erasing characteristics of this optical pickup device are degraded in the case of the deviation of the light spot.
Japanese Laid-Open Patent Application No.4-34740 discloses an optical pickup device in which the above-mentioned beam profile correcting device is formed by a combination of prisms. The use of the prisms in the optical system of an optical pickup device causes the laser beam to be refracted in a certain manner. The design of optical paths of the elements in the optical pickup device will be complicated and restricted due to the refraction of the laser beam by the prisms. Hence, the beam profile correcting device as in the optical pickup device of the above publication is significantly inconvenient when creating the layout of an optical pickup device which has a simple structure and is inexpensive.
Japanese Laid-Open Patent Application No.8-55363 discloses an optical pickup device in which a single objective lens is provided for a first light source and a second light source in a shared manner. A condenser lens which is configured to suitably access the first optical disk (or the DVD) is provided. In the optical pickup device of the above publication, the object distance of the light beam entering the objective lens is selectively changed for accessing the first optical disk (or the DVD) and for accessing the second optical disk (or the CD). When the second optical disk (or the CD) is accessed by using the condenser lens and the objective lens, the laser beam entering the objective lens is divergent. The wave aberration of the objective lens when it is shifted in a radial direction of the second optical disk due to the tracking control, is considerably degraded. In such a case, the accessing characteristics of the optical pickup device for the second optical disk will be significantly lowered.